I plan on doing an open baffle with a Fostex FE206En full range driver and a Kef B200 woofer in a woofer box below the open baffle. I need some help with the design and plans for the box and the baffle please. I intend on using 18mm MDF for the build.

The spec sheets appear to be using different methods for determining sensitivity, maybe someone who's good with dB math can tell if they're equivalent.
First thing to do is decide how wide a baffle you can live with...

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Impedance varies with frequency, use impedance plots of your drivers and make crossover calculations using the actual impedance of the driver at the crossover frequency

The B200 in a "closed" box and the FE206 on a panel.
A closed box (50L). But I recommend an aperiodic BOX with a variovent . You can make a TL line too
You should have 84dB of sensitivity with the crossover.

The F206 on a panel 25cm large (The woofer box has the same width). This gives you a corner frequency near 688Hz.

The crossover is simple, crossover frequency near 500Hz. This could be a starting point
L on the woofer 4.7mH . Could be a 12dB with 47uF in parallel, you can try.
R and C on the FE206 (22 ohms / 10uF )

The B200 in a "closed" box and the FE206 on a panel.
A closed box (50L). But I recommend an aperiodic BOX with a variovent . You can make a TL line too
You should have 84dB of sensitivity with the crossover.

The F206 on a panel 25cm large (The woofer box has the same width). This gives you a corner frequency near 688Hz.

The crossover is simple, crossover frequency near 500Hz. This could be a starting point
L on the woofer 4.7mH . Could be a 12dB with 47uF in parallel, you can try.
R and C on the FE206 (22 ohms / 10uF )

Hope this helps.

I will be bi-amping the set, with a tube amp for the Fostex and a hybrid for the woofer.